1. Technical Field
The present invention relates to a gearbox for motor vehicles, which is arranged between a basic gearbox and a transmission leading to driving wheels. The gearbox includes an input shaft from the basic gearbox, an output shaft to a transmission, and a planetary gear arranged between the input shaft and the output shaft. The ring gear of the planetary gear is axially displaceable. A first coupling ring for engaging a high-range mode, and a second coupling ring for engaging a low-range mode are provided. Utilizing the coupling rings, the ring gear can alternatively engage at least one synchronizing means with at least one friction surface. The synchronizing means is configured to synchronize the rotational speed difference between the ring gear and one or other coupling ring by interaction with a corresponding friction surface arranged on each coupling ring. The gearbox is intended to be capable of shifting between two gearings and thus, together with the basic gearbox, doubling the total number of gearing possibilities.
2. State of the Art
In transmission systems for heavy-duty vehicles, for example trucks and buses, it is known to connect an extra gearbox to the basic gearbox of the vehicle for the purpose of doubling the number of gearing possibilities. Such a gearbox is usually referred to as a range gearbox or an auxiliary gearbox. The range gearbox usually comprises (includes, but is not limited to) a planetary gear, by means of which the gearing can be changed between a high-range mode and a low-range mode. In the low-range mode, use is made of the gearing in the planetary gear, whereas, in the high-range mode, no transmission takes place through the planetary gear. In order to facilitate shifting between high-range mode and low-range mode, it is known to design such planetary gears with synchronizing means, usually comprising synchronizing rings, spring means for pre-synchronizing and blocking means to prevent engagement before synchronous rotational speed has been achieved.
In order to reduce the number of component parts and also from the point of view of power distribution, it has been found to be advantageous to use the ring gear of the planetary gear as a coupling sleeve. The ring gear can, by axial displacement, and after synchronization of the rotational speed difference between the ring gear and coupling rings, be brought into engagement with the coupling ring concerned on either side of the planetary gear. In this way different gearings are obtained. An example of an axially displaceable ring gear is shown in SE 514231. In that case, the ring gear is provided at each axial end with synchronizing means, and the internal teeth of the ring gear are extended so as to be capable of being coupled together with a corresponding coupling ring with coupling teeth arranged on each side of the ring gear. The ring gear and the synchronizing means on each side thus occupy more space in the axial direction.
When the ring gear in SE 514231 is coupled together with the gearbox casing, low-range mode is obtained, and the gearing in the planetary gear is used.
In order to obtain a shorter shifting time when synchronizing the rotational speed for the low-range mode, a greater synchronizing torque is required compared with synchronization for the high-range mode. A large diameter of the low-range synchronizing device is thus desirable in order to obtain a greater synchronizing torque. The diameter refers to the position of the synchronizing friction surfaces. In the range gearbox of SE 514231, the inside diameter of the ring gear limits the possibility of a large synchronizing diameter and thus a great synchronizing torque. Another disadvantage of known devices is that the overall length is in many cases too great and that double sets of annular springs with associated synchronizing rings are required.
The object of the present invention is to make synchronization for the low-range mode act on as large a diameter as possible in order to obtain more rapid shifts and also to shorten the overall length of the gearbox and reduce the number of component annular springs and synchronizing rings, while retaining good functioning. The object is also, when the axial displacement of the ring gear takes place, to separate friction surfaces in the synchronizing device with a relative speed difference; that is to say, the synchronizing device which is not functioning at the time, and in this way reduce the friction losses.